Breast cancer is the second most common cancer and the second leading cause of cancer death among women in West Virginia/Central Appalachia. About one out of every three cancer deaths are linked to excess body weight and Appalachia and West Virginia have some of the highest rates of obesity in the country. Obese women (especially in postmenopausal women) have higher rates of breast cancer incidence, are less responsive to cancer therapy and have worse clinical outcomes than non-obese women. Leucine is an essential amino acid that is elevated in obesity and is required for the activation of Mechanistic Target Of Rapamycin (mTOR), which is kinase that is critical for tumor growth. Breast cancer cells absorb extracellular leucine through L-Type Amino Acid Transporter 1 (LAT1). The expression of LAT1 is higher in breast cancer compared with normal breast tissue, and its expression is more highly expressed in more advanced cancers, indicating that it is associated with breast cancer progression. Identifying the regulation and function of LAT1- leucine signaling in breast cancer in obesity should foster new ways to suppress this disease in obesity. Our preliminary studies have uncovered that the adipocyte secretome upregulates the expression of LAT1 and mTOR1 activity in human breast cancer cells. Increases in the transcription and activity of LAT1 are driven by the aryl hydrocarbon receptor (AHR) and the estrogen receptor (ER). Preventing leucine uptake has a marked inhibitory effect on breast cancer cell colony formation, which supports our hypothesis that leucine transporters are critical for breast cancer cell survival and proliferation. These findings provide the basis for our proposal that will establish the regulation and function of LAT1-leucine in breast cancer in obesity and its requirement for mTOR1 activity and tumor progression. In Aim 1, we will decipher the signaling pathways that activate leucine-mTOR signaling in breast cancer in response to obesity-associated hormone levels. Aim 2 will focus on elucidating the intracellular mechanisms by which obesity in vivo promotes breast cancer development and progression over long periods of time. These studies should lead to the development of new therapies that inhibit leucine-stimulated induction on mTOR1 to suppress breast cancer in obesity. In addition to increasing our understanding of obesity and breast cancer, participation in this COBRE project will allow me to receive intensive career and scientific mentoring while generating important novel data to attain my goal of becoming a successful independent investigator with NIH R01 funding.